Two sea lions caught preying on endangered chinook salmon in Washington state had faced a federal death penalty.

Reporting from Chicago — In March, a couple of plump, 900-pound California sea lions showed up at the Bonneville Dam, which spans the Columbia River between Washington and Oregon, 146 miles inland from the Pacific Ocean.

Their mission: to gorge themselves on a feast of endangered chinook salmon laboring to get over the dam's fish ladder.

The two had been caught before and branded as recidivist malefactors by wildlife officials, who have spent decades and billions of dollars trying to protect the salmon. So when captured again March 18, the sea lions faced a federal death penalty.

But a mercy phone call to the Shedd Aquarium saved their hides.

Now known as Biff and Otis, the two have a home in Chicago, where they are being tutored to become main-stage stars at the Shedd's Oceanarium marine mammal show. It turns out their reprieve was a pretty good deal for their appetites too.

"They lived the life of Riley, catching fish at the dam," said Ken Ramirez, the Shedd's vice president of animal collections. "But it was even easier to get fish from us, so they weren't upset at all to be hanging around with us."

The first thing visitors notice about Biff and Otis, now slimmed down to their nonbreeding weight of 650 pounds, are the letter C and identification numbers branded prominently on their backs. Otis is 507. Biff is 700.

The Bonneville, built in 1937, was the first of several hydroelectric dams to go up on the Columbia and its tributary rivers. But the dams and a century of overfishing have taken their toll on ocean fish, such as steelhead trout and salmon, which have to return to spawning grounds hundreds of miles upriver to lay and fertilize their eggs.

California sea lions also were over-hunted -- their numbers fell to 10,000 in the 1950s. But they have made a dramatic rebound, and 300,000 now dot the Pacific Coast from Mexico to Alaska.

In the 1980s, they began following fish on spawning runs up the Columbia. By 2002, big numbers were going all the way to the Bonneville, feasting around the fish ladder next to the 197-foot-high dam.

The dam attracts only male sea lions, which arrive for the January-May salmon spawning runs to fatten themselves up before returning to breeding beaches in California and Mexico in June for their own mating season.

At the dam, officials tried for several years to chase off the seals with nonlethal efforts, such as scaring them with boats, underwater firecrackers and rubber shotgun pellets -- all of which were ineffective. In 2008, Oregon, Washington and Idaho officials got federal authority to kill the seal lions by lethal injection.

"This has been fairly controversial," said Craig Bartlett, spokesman for Washington state's Fish and Wildlife Department.